1. Technical Field
The present invention relates to gantry systems, and more particularly to apparatus and methods for guiding cables around a rotating gantry of a nuclear medicine camera apparatus.
2. Discussion of Related Art
Medical diagnostic imaging began with the discovery of x-rays by W. C. Roentgen in 1895 and today includes radiography, nuclear medicine imaging, ultrasound imaging, computed tomographic imaging, and magnetic resonance imaging. In general, the goal of each type of medical imaging is to provide a spatial mapping of a parameter, feature, or biological process within a patient.
In radiology and computed tomography, a source of x-rays is transmitted through the patient onto a suitable detector such as a film or a plate. The detector measures the intensity distribution of the incident beam of x-rays and provides an image representing the attenuation of the radiation resulting from the absorption and scattering within the patient's body.
Nuclear medicine involves injection of a radiopharmaceutical into a patient and measurement of the intensity distribution of gamma radiation emitted from the patient's body. Radiopharmaceuticals are formed by attaching a radioactive tracer to a pharmaceutical that is known to preferentially accumulate in the organ of interest. Thus, the radiation pattern is a measure of blood flow, metabolism, or receptor density within the organ of interest and provides information about the function of the organ. Either a single projection image of the radiation pattern may be taken (planar imaging) or many projection images may be acquired from different directions and used to compute the three dimensional emission distribution (single photon emission computed tomography, or SPECT). Radiation-imaging systems used in nuclear medicine are often referred to as “gamma” cameras because emitted gamma photons are detected by the system.
Nuclear or gamma cameras have included a detector head which receives radiation emanating from the patient. The detector head includes a flat scintillation crystal which converts incident radiation to flashes of light. Internal electronics convert each flash of light into an electric signal indicative of the location and energy of the received incident radiation event. Collimators are commonly mounted on the face of the detector head such that the scintillation crystal only receives radiation coming generally straight toward it. Generally, the collimators are a series of lead vanes arranged in a grid pattern. The height of the vanes and their spacing control the angle at which received radiation may differ from perpendicular. Different collimators are provided for different types of medical procedures.
Various mechanical gantry systems are known in the art. Many of the known gantry systems enable the detector head, typically on the order of several hundred pounds, to be positioned at a selected location over the patient. Commonly, the gantry is also motor-controlled such that the detector head can be moved continuously or intermittently either (i) longitudinally along the length of the patient or (ii) circumferentially around the patient. Some gantry systems also support a second detector head which is positioned diametrically opposed, around the patient, from the first detector head.
For axial scans, the camera head and the patient are moved relative to each other. In some of the prior art systems, the entire camera gantry was mounted on floor rails to be moved longitudinally relative to a stationary patient. Other prior art gantry systems are mounted on rails to move relative to a stationary patient. Typically, these gantry systems are supported on three or four wheels.
One of the design challenges in a nuclear medicine camera is how to guide the electrical cables from the stationary outer structure of the system to the rotating gantry and detectors, which typically have approximately 1.5 revolutions of travel as they rotate around the patient. The method used on the Siemens E.CAM system, for example, is to wrap the cables one way around the rotating inner diameter, then unwind and wrap the cables the other way around the same diameter as the detector head rotates in the opposite direction. As the cable is unwrapped, approximately seven (7) feet of slack must be taken up by a system of pulleys and a sliding spring-loaded accumulator/tensioner wheel. Methods of this type, while effective, tend to require a relatively large number of moving parts and thus can be expensive to manufacture and maintain. Plastic cable guide type chains are cost effective and are commonly used to guide cables on moving systems. However, since the chain would need to rotate all the way around the gantry ring, it would need a support system to resist falling for most of its travel.
Another challenge is how to individually sense both ends of travel for a rotary axis with approximately 1.5 revolutions of motion. If limit switches are simply used to sense a flag on the rotating gantry, they will activate every time the flag goes by (once per 360 degree revolution of the camera), which would include “on” signals in the middle of a 540 degree range of travel. This method can be made to work, however, if the switch logic is processed at a high level by the machine control software. Using the system drive motor's servo-controlled position or even a separate encoder (or potentiometer, as with E.CAM) is another method, but this too relies on high level machine control software processing the signal to determine the ends of travel. A potentially more robust method would be to use a system with two limit switches that turn on only when the system is at each respective end of travel. Thus simple on/off logic could be used to always stop motion whenever a limit switch is activated.
The present invention contemplates a new and improved gantry system which overcomes the above-referenced problems and others.